From the aspect of protecting environment not only in regions but also in the entire earth, a ceramic honeycomb filter constituted by a ceramic honeycomb structure having both ends on exhaust gas-introducing and exiting sides sealed alternately, is used to remove carbon-based particulates from an exhaust gas discharged from diesel engines.
FIG. 3 is a cross-sectional view showing a conventional honeycomb filter. In a ceramic honeycomb filter 30 having such structure, an exhaust gas containing particulates flows into flow paths 37 of a honeycomb structure 31 that are open at an inlet 31a of the honeycomb structure 31, passes through cell walls 36 constituted by porous ceramics, and exits from an outlet 31b via adjacent flow paths. During this process, particulates in the exhaust gas are captured by pores (not shown) of the cell walls 36. As particulates are continuously captured in the honeycomb filter 30, the pores of the cell walls 36 are clogged, resulting in drastic decrease in a capturing function and thus increase in a pressure loss. The reduction of an engine output ensues. In view of this, a technology was proposed to burn particulates accumulated in the honeycomb filter 30 by an electric heater, a burner, a microwave-generating means, etc. to regenerate the honeycomb filter 30.
However, when particulates captured in a honeycomb filter having a conventional structure are burned by an electric heater or a burner, only small amounts of particulates are attached in an upstream region, so that heat generated by the burning of particulates is not sufficient to keep the self ignition of the attached particulates, resulting in difficulty in the regeneration of a downstream region. Also, when regeneration is carried out by a microwave system as shown in JP 59-126022 A, for instance, a portion of the filter near the air supply side is cooled by air supplied, so that the temperature elevation of particulates is hindered, resulting in narrowing of a region in which the burning of particulates occurs and thus difficulty in effective regeneration of the entire honeycomb filter. As a result, when air necessary for the burning of particulates is supplied from an exhaust gas inlet side to continuously capture the particulates, unburned particulates are so accumulated near an end surface of the filter that flow paths open on an exhaust gas inlet side are clogged, thereby losing a particulates-capturing function and resulting in extreme decrease in a filter-regenerating function.
To solve these problems, JP 59-28010 A discloses a honeycomb filter having a space disposed between plugs positioned on an exhaust gas inlet side and the end surfaces of flow paths on the exhaust gas inlet side. FIG. 4 is a cross-sectional view showing a honeycomb filter 40 described in JP 59-28010 A. Because the honeycomb filter of FIG. 4 comprises a space 49 between plugs 48a positioned on an upstream side of the flow paths 47 and the end surfaces of inlets 41a of the honeycomb structure 41, particulates in an exhaust gas are captured by partition walls in the space 49, so that larger amounts of particulates are attached near an upstream region. Accordingly, particulates are burned by a heating means mounted on the inlet side of the filter, so that the burning of particulates can be easily conducted in a downstream region.
Japanese Patent 2,924,288 discloses a honeycomb filter-regenerating apparatus comprising a heating chamber mounted onto an exhaust pipe of an engine, a means for generating microwaves that are supplied to the heating chamber, a honeycomb filter contained in the heating chamber for capturing particulates in an exhaust gas, and a means for supplying air to the heating chamber. FIG. 5 is a cross-sectional view showing a honeycomb filter 50 in the honeycomb filter-regenerating apparatus of Japanese Patent 2,924,288. The honeycomb filter 50 is constituted by a honeycomb structure 51 having a large number of flow paths 57 partitioned by cell walls 56 enclosed by an outer wall 55, inlet portions 51a and outlet portions 51b being alternately sealed by plugs 58a, 58b, the plugs 58a being positioned inside the end surfaces of the inlet portion 51a to constitute a heat dissipation-preventing means 59. In this honeycomb filter, when the captured particulates are heated by microwaves, the particulates reach their burning temperature in a short period of time because of the heat dissipation-preventing means 59. Incidentally, the arrow X shows an exhaust gas-flowing direction.
In any of the above ceramic honeycomb filters, as shown in FIGS. 4 and 5, to efficiently regenerate the entire honeycomb filter, plugs on an exhaust gas inlet side are disposed inside the filter separate from an inlet end surface of the filter. It has been found, however, that the actual use of the honeycomb filters having such structures causes the following problems.
As shown in FIG. 6(a), in the honeycomb filter 40 described in JP 59-28010 A, the plugs 48a on the inlet side are formed as follows: First, the end portions of flow paths needing no plugs are sealed with a wax 61, and the inlet end portions 41a of the honeycomb structure 41 are then immersed in the plug-forming slurry 60 to cause the slurry 60 enter into the flow paths 47a not plugged with a wax. Because the honeycomb structure 41 is made of a porous ceramic and thus water-absorptive, an upper portion of the slurry entering into the flow paths 47a is deprived of water by the cell walls and thus solidified, while a lower portion of the slurry remains in the form of a slurry for the lack of the cell walls that can remove water. This honeycomb structure is turned upside down as shown in FIG. 6(b), so that a slurry remaining in the flow paths spontaneously falls onto the solidified slurry portion to form plugs 48a. The position of the inlet-side plugs is determined by the height of the slurry entering into the cells.
However, the actual trial of the inventors to fill the flow paths 47a with the slurry 60 has revealed that because water is absorbed by the cell walls in contact with the slurry regardless of the position of the slurry, solidification starts simultaneously in the upper and lower portions of the slurry. It is thus difficult to cause solidification only in the upper portion of the slurry, and the entire regions of the cells on an upstream side of the inlet-side plugs are likely sealed by the slurry. Accordingly, it is difficult to provide a space as shown in FIGS. 2 and 9–15 of JP 59-28010 A in cell portions on an upstream side of the inlet-side plugs. This tendency is remarkable, for instance, when the inlet-side plugs are disposed at positions separate from the end surface of the ceramic honeycomb by 10 mm or more. Accordingly, this honeycomb filter cannot exhibit functions of capturing particulates and preventing heat dissipation as expected, failing to efficiently carry out the regeneration of the entire filter, and resulting in a large pressure loss.
Also disclosed is a method for integrating ceramic chips inserted into the honeycomb structure as inlet-side plugs 48a with cell walls by sintering. However, because it is difficult to make the ceramic chips completely equal to an extrusion-molded honeycomb structure in material properties such as a thermal expansion coefficient, etc., a gap inevitably occurs between the ceramic chip and the cell walls due to thermal expansion and shrinkage accompanying the sintering, making it likely that a particulates-capturing efficiency decreases, that the ceramic chips detaches from the cell walls, and that the cell walls are broken. In addition, even if the ceramic chips are integrated with the cell walls, the ceramic chips (plugs) are likely to detach from the filter by thermal shock when particulates are burned in the filter, because of their difference in a thermal expansion coefficient.
A honeycomb structure used as a particulates-capturing filter usually has an extremely small cell size (cell pitch), for instance, 2.54 mm for 100 cpsi and 1.47 mm for 300 cpsi. Accordingly, it is difficult to embed the ceramic chips in the cells accurately, and when the inlet-side plugs are disposed separate from the end surface of the ceramic honeycomb, for instance, by 10 mm or more, it is difficult to dispose all ceramic chips at proper positions. If the inlet-side plugs were not accurately positioned, a space on an upstream side of the inlet-side plugs would have non-uniform volume, failing to efficiently regenerate the entire honeycomb filter, and resulting in different pressure losses among the filters. With respect to Japanese Patent 2,924,288, it fails to disclose a specific method for forming plugs 58a at the inlets 51a. 